1. Field of the Invention
The present invention relates to an electrodeionization apparatus which can reduce electric resistance, thereby minimizing power consumption, and to an electrodeionization method using such an apparatus. This electrodeionization apparatus can be used for production of deionized water in such applications as electronics, pharmaceutical, nuclear or fossil-fuelea power generation, food and beverage, and laboratory.
2. Description of the Related Art
One conventional method for producing deionized water is to pass the water to be treated through ion exchange resins. In this method, the ion exchange resins must be chemically regenerated when they become exhausted. In order to eliminate this troublesome operation, an electrodeionization (hereinafter abbreviated to EDI) method has been developed and commercialized which does not require chemical regeneration.
FIG. 5 is a cross sectional view of a typical conventional electrodeionization apparatus. As shown in the figure, a series of desalination chambers 104 are formed by alternately providing cation exchange membranes 101 and anion exchange membranes 102 with spaces in between, and by filling every other space with ion exchange materials 103. One side (front side) of each desalination chamber, from where water to be treated is fed, is filled with an anion exchange resin 103a and the other side (back side) of the desalination chamber, where treated water flows out, is filled with a mixed bed 103b of cation and anion exchange resins. The sections adjacent to desalination chambers 104 which are formed by anion exchange membrane 102 and cation exchange membrane 101 and which are not filled with ion exchange material 103 act as concentrate chambers 105 where concentrate water flows.
As shown in FIG. 6, a deionizing module 106 is formed by a cation exchange membrane 101, an anion exchange membrane 102, and ion exchange materials 103 filling the space between these ion exchange membranes.
Specifically, one side of a frame 107 is sealed with a cation exchange membrane 101. The upper side (front side) of the interior of the frame 107 is filled with an anion exchange resin 103a and the lower side (back side) of the frame interior is filled with a mixed ion exchange resins 103b. The other side of the frame 107 which is then sealed with an anion exchange membrane 102. Because ion exchange membranes 101 and 102 are soft and flexible, when frame 107 is filled with the ion exchange materials 103 and sealed with ion exchange membranes on both sides, in a typical frame 107, a plurality of vertical ribs 108 are provided to prevent nonuniform filling of ion exchange materials 103 due to curving of ion exchange membranes. Although not shown in the figure, inlets for the water to be treated are provided at the upper side of the frame 107 and outlets for the treated water are provided at the lower side of the frame 107.
FIG. 5 shows a plurality of these deionizing modules 106 provided in parallel with spacers (not-shown) in between. A cathode 109 is provided on one side of the parallel deionizing modules 106 and an anode 110 is provided on the opposite side of the parallel deionizing modules 106. The spacers are provided between deionizing modules 106 and concentrate chambers 105. A separating membrane such as a cation exchange membrane 101, an anion exchange membrane 102, or a diaphragm which does not have any ion exchange functionality is provided externally on both of the outermost concentrate chambers 105, as necessary. A cathode chamber 112 and an anode chamber 113 are provided in the sections separated by the above-mentioned separating membranes, which sections come in contact with the cathode 109 and anode 110. As can be seen, in such a conventional EDI apparatus, the number of the concentrate chambers is larger than the number of desalination chambers by one.
A deionized water producing process using such an EDI apparatus is explained referring to FIGS. 4 and 5, where FIG. 4 schematically shows a relationship between the desalination and concentrate chambers. In FIG. 4, cathode chamber 112 and anode chamber 113 are separated from concentrate chambers 105 by cation exchange membrane 101. Specifically, a direct current is applied between the cathode 109 and anode 110. Water to be treated is fed from the water to be treated supply line 111 and concentrate water is fed from the concentrate water supply line 115. Electrode water is supplied from electrode water supply lines 111 and 117. The water to be treated which has been fed from the water to be treated supply line 111 flows through the desalination chambers 104. Anions such as chloride and sulfate ions in the water are removed when the water flows through the anion exchange resin 103a in the front side and then cations such as magnesium and calcium ions are removed when the water flows through the downstream mixed ion exchange resins 103b of the cation and anion exchange resins. The concentrate water fed from the concentrated water supply line 115 flows upward through each concentrate chamber 105, receives impurity ions via the cation exchange membrane 101 and anion exchange membrane 102 and is discharged from the concentrate water discharge line 116 as concentrate water containing concentrated impurity ions. The electrode water supplied from the electrode water supply lines 117 and 117 is discharged from electrode water discharge lines 118 and 118. Thus, deionized water can be produced in the deionized water discharge line 114.
There have been various attempts to reduce the electric resistance of this type of EID apparatus in order to reduce the amount of electric power consumption when such an apparatus is used to remove impurity ions from water to be treated. However, because the filling method and amount of the ion exchange materials used in the desalination chambers depend on the desired quality of treated water, there are restrains on how much the electric resistance at the desalination chamber can be reduced, and measures have often been taken for reducing the electric resistance at the concentrate chambers. For example, Japanese Patent Laid-Open Publication No. Hei 9-24374 discloses a method for reducing the electric resistance at the concentrate chambers by adding electrolytes thereto. A method for reducing electric resistance in the concentrate chamber by circulating concentrate water to promote an increase in its electric conductivity has also been proposed.
However, in the method for reducing the electric resistance in the concentrate chambers by adding electrolytes thereto, a pump for supplying the electrolytes to the concentrate chambers, a chemical storage tank, and supply pipes must be provided, and therefore, both installation area and costs increase. Moreover, chemicals must periodically be supplied and managed, causing a problem that considerable personal attention is required, even though the apparatus is referred to as a continuous electrodeionization unit. The method for reducing the electric resistance in the concentrate chambers by circulating concentrate water, thereby increasing the electrical conductivity in the concentrate chambers has also a disadvantage in that hardness components such as calcium and magnesium within the concentrate water also become highly concentrated and form scales which increase electric resistance.